Data of a graph structure with nodes and edges representing connection relationships between the nodes (hereinafter, which may be simply referred to as “graph data”) is often used to express a real world model on a computer. Graph data is generally described in accordance with a standardized format such as the resource description framework (RDF), and stored in a database termed a “graph repository”. A part or whole of the graph data stored in a graph repository is called by various kinds of application programs for use. Companies in the industry have conducted research and development on software for handling such graph data.